7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Fast response of turbulence and heat pulses to thermal perturbations
Naoki Kenmochi1, Katsumi Ida1, Tokihiko Tokuzawa1, Ryo Yasuhara1, Hisamichi Funaba1,Hiyori Uehara1, Daniel J Den Hartog2, Ichihiro Yamada1, Mikiro Yoshinuma1, Hiroe Igami1,and Ryoma Yanai1
1
National Institute for Fusion Science2
Wisconsin Plasma Physics Laboratory, University of Wisconsin-Madisone-mail (speaker): kenmochi.naoki@nifs.ac.jp

Plasma transport cannot be explained by local models
alone, and the effects of non-local transport must betaken into account. In particular, avalanche phenomena
and turbulence spreading have been recognized as
causes of non-local transport. However, observations ofthese phenomena are limited due to measurement
limitations, and both experimental and theoreticalunderstanding of these phenomena are inadequate. This
study reports the new findings of turbulence and heatpropagation phenomena that cannot be explained by
the existing avalanche and turbulence spreading models
in the Large Helical Device (LHD) experiments.
In order to induce large observable turbulence
spreading phenomena, a collapse of the electroninternal transport barrier (e-ITB) is targeted. In theLHD, both turbulence and thermal pulses are generated
near the foot of the e-ITB and propagate to theperipheral region faster than the diffusion time whenthermal avalanche phenomena accompany the e-ITB
collapse. The propagation velocity of the turbulence
pulses is about 10 km/s, which is faster than thepropagation velocity of the heat pulse of about 1.5 km/s
[1]. Existing models predict that both heat andturbulence propagate at about 1 km/s, but theturbulence pulse propagates more than an order ofmagnitude faster than predicted. Although thesimultaneous propagation of the turbulence and heatpulses has been reported in a number of experiments, itis thought that the phenomenon observed in the LHD
is due to a different mechanism.
The modulated electron cyclotron heating (ECH)
experiments, in which the induced time width of theheat pulse was varied by changing the time width of theECH, were performed to systematically study theeffects of the time scale of the turbulence and heatpulses on their propagation velocity (See Figs. 1(a)(b)).
As shown in Fig. 1(c), the results showed that thepropagation velocity of the heat and turbulence pulses
is faster at shorter pulse widths. The smaller thetimescale of the thermal pulse, the larger the deviation
between the velocity of the turbulent pulse and thethermal pulse, with the turbulent pulse being much
faster than the thermal pulse. This result indicates theexistence of a phenomenon that cannot be explained byexisting models in which heat and turbulence propagate
simultaneously, and provides essential insight into thephysical mechanism of non-local transport.

References[1] N. Kenmochi et al., Scientific Reports 12, 6979
(2022)

Figure 1. (a)(b) The time evolution of the electron temperature (Te) profiles measured by ECE with thevariation normalized between 0 and 1. The time evolutions of ECH injection powers are shown at the bottomof the figures. The pulse widths of ECH in (a) and (b) are 4 ms and 16 ms, respectively. (c)The relationship ofradial propagation velocity of turbulence (blue) and heat (red) pulses to the time scale of heat pulses.